An avalanche photodiode is important as an optical detector in an optical communication system, because an avalanche photodiode is a semiconductor photodetector having high sensibility and high operation speed. There are several avalanche photodiodes which have been realized until now for an optical communication system using a silica optical fiber for transmitting a light having a wavelength of 1.3 .mu.m or 1.55 .mu.m, one of which is an avalanche photodiode using Ge or an avalanche photodiode having a hetero-junction structure of InP/InGaAs system including an optical absorption layer of InGaAs which is lattice matched to InP and an avalanche multiplication layer of InP.
These avalanche photodiodes are required to have a high S/N ratio for high quality optical communications. The S/N ratio depends on several factors, one of which is a noise factor. The noise factor depends on the ratio of ionization rates of electrons and holes of a material forming an avalanche region of an avalanche photodiode. The ratio of the ionization rates is required to be high for a high S/N ratio for avalanche photodiodes. However, the ratio of the ionization rates of an avalanche photodiode including an avalanche multiplication layer of Ge is approximately 1, and that of an avalanche photodiode including an avalanche multiplication layer of InP/InGaAs is up to 3. Therefore, materials having a high ratio of the ionization rates have been sought.
A first conventional avalanche photodiode having an avalanche layer consisting of InGaAs/InAlAs superlattice has been disclosed on page 467 of Electronics Letters, vol. 16, 1980 by Chin. R, et al and on page 597 of Applied Physics Letters, vol. 47, 1985 by Capasso. F, et al. In this avalanche photodiode, conduction band edge discontinuity energy (approximately 0.5 eV) between InGaAs and InAlAs is given to electrons which run through the avalanche multiplication layer from InAlAs to InGaAs at a high speed, so that the ionization rate of the electrons increases to increase the ratio of the ionization rates. The ratio of the ionization rates of an avalanche photodiode having an avalanche multiplication layer consisting of InGaAs/InAlAs was measured on page 993 of Applied Physics Letters, vol. 55, 1989 by Kagawa. T, et al, however, it has been found that the ionization rate of electrons increases dependence on the superlattice structure.
A second conventional avalanche photodiode includes an avalanche multiplication layer having InGaAsP/InAlAs superlattice structure disclosed in Japanese Preliminary Publication (Kokai) No. 2-137376 (corresponding to U.S. Pat. No. 4,982,255 to Tomita).
In this avalanche photodiode, well layers consist of InCaAsP quaternary system mixed crystal which has the forbidden bandgap energy of 1 to 1.2 eV due to its small discontinuity of the valence band.
According to the conventional avalanche photodiodes, however, there is a disadvantage in that noise characteristic is not sufficient. In the first conventional avalanche photodiode which includes an avalanche multiplication layer of InGaAs/InAlAs superlattice structure, the forbidden bandgap energy of InGaAs is approximately 0.75 eV, so that effective mass of electrons in InGaAs is as small as 0.04 times of that of free electrons. Therefore, a tunnel current flows on condition of a high electric field higher than 200 KV/cm, and a large dark current flows on condition of a high electric field higher than 300 KV/cm which is necessary for avalanche multiplication.
In the second conventional avalanche photodiode, it is difficult to fabricate an avalanche multiplication layer to provide a plurality of layers including P and layers not including P alternately one layer on the other layer with high reproducibility.